A light-emitting element using an organic compound as a light-emitting body, which has features such as thinness, lightness, high-speed response, and DC drive at a low voltage, has been examined to be applied to a next-generation flat panel display or next-generation lighting. In particular, a display device in which light-emitting elements are arranged in matrix is considered to have advantages in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
A light-emitting mechanism of a light-emitting element is thought as follows: when voltage is applied between a pair of electrodes with an EL layer interposed therebetween, electrons injected from a cathode and holes injected from an anode are recombined at emission centers in the EL layer to form molecular excitons, and the molecular excitons release energy and emit light when relax to the ground state. Singlet excitation and triplet excitation are known as excited states, and it is considered that light emission can probably be achieved through either of the excited states.
An EL layer included in a light-emitting element has at least a light-emitting layer. In addition, the EL layer can have a stacked-layer structure including a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, or the like, in addition to the light-emitting layer.
Further, as a material having semiconductor characteristics, metal oxides attracts attention. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. Thin film transistors in which a channel formation region is formed of such metal oxides having semiconductor characteristics are already known (Patent Documents 1 and 2).
Furthermore, a TFT including an oxide semiconductor has high field-effect mobility. Thus, the TFT can be used to form a driving circuit of a display device or the like.